Mine

SOURCES += main.c
TARGET = calccomms
TICABLES_MINVERSION = 0.1.3
HAVE_TICABLES = $$system(pkg-config --atleast-version=$$TICABLES_MINVERSION ticables2 && echo yes || echo no)
!equals(HAVE_TICABLES,yes):error(libticables2 $$TICABLES_MINVERSION or higher required.)
PKGCONFIG_CFLAGS += $$system(pkg-config --cflags ticables2)
LIBS += $$system(pkg-config --libs ticables2)
CXXFLAGS = $$(CXXFLAGS)
isEmpty(CXXFLAGS) {
   debug {
     CXXFLAGS = -Os -g
   } else {
     CXXFLAGS = -Os -s -fomit-frame-pointer
   }
 }
QMAKE_CXXFLAGS_DEBUG = $$CXXFLAGS -Wno-non-virtual-dtor $$PKGCONFIG_CFLAGS
QMAKE_CXXFLAGS_RELEASE = $$CXXFLAGS -Wno-non-virtual-dtor $$PKGCONFIG_CFLAGS

CFLAGS = $$(CFLAGS)
isEmpty(CFLAGS) {
   debug {
     CFLAGS = -Os -g
   } else {
     CFLAGS = -Os -s -fomit-frame-pointer
   }
 }
QMAKE_CFLAGS_DEBUG = $$CFLAGS $$PKGCONFIG_CFLAGS
QMAKE_CFLAGS_RELEASE = $$CFLAGS $$PKGCONFIG_CFLAGS